In a normal type of quadrupole mass spectrometer, various kinds of ions created from a sample are introduced into a quadrupole mass filter, which selectively allows only ions having a specific mass-to-charge ratio to pass through it. The selected ions are detected by a detector to obtain an intensity signal corresponding to the amount of ions.
As commonly known, a quadrupole mass filter normally consists of four rod electrodes arranged parallel to each other around an ion-beam axis, and a composite voltage composed of a direct-current (DC) voltage and a radio-frequency (RF) voltage (AC voltage) is applied to each of the four rod electrodes. The mass-to-charge ratio of the ions which are allowed to pass through a space extending along the ion-beam axis of the quadrupole mass filter depends on the RF voltage (amplitude) and the DC voltage applied to the rod electrodes. Accordingly, by appropriately setting the RF and DC voltages according to the mass-to-charge ratio of an ion to be analyzed, it is possible to selectively allow an intended kind of ion to pass through the filter and be detected. It is also possible to vary each of the RF and DC voltages applied to the rod electrodes over a predetermined range so that the mass-to-charge ratio of the ion passing through the quadrupole mass filter will change over a predetermined range, and to create a mass spectrum based on the signals produced by the detector during this process. This is the so-called scan measurement.
A detailed description of the voltage applied to the rod electrodes of the quadrupole mass filter is as follows. Normally, among the four rod electrodes, each pair of rod electrodes facing each other across the ion-beam axis are electrically connected. A voltage U+V cos·ωt is applied to one of the two pairs of rod electrodes, while a voltage −U−V cos·ωt is applied to the other pair of rod electrodes, where ±U and ±V cos·ωt are the DC and RF voltages, respectively. A common DC bias voltage, which may additionally be applied to all the rod electrodes, is disregarded in the present discussion since this voltage basically does not affect the mass-to-charge ratio of the ions that can pass through the filter. For simplicity, the expressions “DC voltage U” and “RF voltage V” will hereinafter be used in place of the aforementioned, exact expressions of U being the voltage value of the DC voltage and V being the amplitude value of the RF voltage.
Normally, when the aforementioned scan measurement is performed, the voltages are controlled so that the voltage value U of the DC voltage and the amplitude value V of the RF voltage will be individually changed while maintaining their ratio (U/V) at a constant value (for example, see Patent Document 1). For example, in a conventional quadrupole mass spectrometer as described in Patent Document 2, the DC voltage U applied to the rod electrodes during the scan measurement is generated by converting voltage-setting data, which is sequentially given from a control CPU, into an analogue voltage by a digital-to-analogue converter. Therefore, the change in the DC voltage U with respect to a change in the mass-to-charge ratio will be approximately linear, as shown in FIG. 6B. Due to this relationship, the DC voltage U is used as a controlling factor for adjusting the mass-resolving power, which is one of the essential capabilities of mass spectrometers. The principle of this adjustment is hereinafter briefly described by means of FIGS. 7A and 7B, which are stability diagrams based on the stability condition for the solution of a Mathieu equation.
The stability region S, in which an ion can exist in a stable state in the quadrupole electric field surrounded by the rod electrodes (i.e. in which an ion can pass through the quadrupole mass filter without being dispersed during its flight), is a region surrounded by a nearly triangular frame as shown in FIGS. 7A and 7B. With an increase in the mass-to-charge ratio, the stability region S increases its area, while moving in the same direction as the increasing direction of the mass-to-charge ratio (rightward). Basically, by changing the DC voltage U so that this voltage U is always included within the stability region S, it is possible to allow ions having desired mass-to-charge ratios to sequentially pass through the quadrupole mass filter. However, the mass-resolving power changes depending on the position at which the line L which shows the change in the DC voltage U with respect to the mass-to-charge ratio traverses the stability region S. This means that, in order to approximately maintain the mass-resolving power at the same level over the entire mass range, it is necessary to change the DC voltage U so that the line L traverses the same relative portion within the stability region S, which always has a similar shape while sequentially changing its position and area. A conventional method for addressing this problem is to regulate two parameters, “gain” and “offset”, so as to control the linear change in the DC voltage U and thereby control the mass-resolving power.
Specifically, the “gain” is a parameter for varying the amount of change in the voltage U with respect to the amount of change in the mass-to-charge ratio. As shown in FIG. 7B, varying the “gain” changes the gradient of the line L which shows the relationship between the mass-to-charge ratio and the voltage U. On the other hand, the “offset” is a parameter for varying the absolute value of the voltage U at the beginning of the change (scan) of the mass-to-charge ratio. Varying the “offset” translates the line L showing the relationship between the mass-to-charge ratio and the voltage U along the axis of voltage U, as shown in FIG. 7A. Conventional quadrupole mass spectrometers have the function of automatically adjusting the two parameters during a calibration process using a standard sample so as to adjust the gradient and position of the line showing the relationship between the mass-to-charge ratio and the voltage U and thereby adjust the mass-resolving power.
In commonly used quadrupole mass spectrometers, the RF voltage V is added to the DC voltage U via a coil and applied to the rod electrodes. As described in Patent Document 1, in many cases, the accuracy of the amplitude value of the RF voltage applied to the rod electrodes is ensured by means of a wave-detection circuit using a diode, by which an envelope of the RF voltage that has passed through the coil is extracted as a wave-detection signal, and the difference between the wave-detection signal and the objective voltage is fed back to an amplitude modulator used for generating the RF voltage. However, as pointed out in the aforementioned document, the output characteristic of the wave-detection circuit in some cases becomes curved, rather than linear, since the linear operation range of diodes used for wave detection is not wide enough. If the operation of the diode is extremely non-linear, the change in the RF voltage V with respect to the change in the mass-to-charge ratio may possibly become significantly curved, as shown in FIG. 6A.
The previous description about the mass-resolving power using the stability diagrams based on the Mathieu equation is only applicable in the case where the relationship between the RF voltage V and the mass-to-charge ratio is linear, similar to the relationship between the DC voltage U and the mass-to-charge ratio. If the relationship between the RF voltage V and the mass-to-charge ratio is non-linear, the uniformity of the mass-resolving power within a range of mass-to-charge ratio will deteriorate.
FIGS. 8A-8C are examples of actually measured mass spectra covering a range from a low mass (m/z168) to high mass (m/z1893) for different values of “gain” and “offset.” In the example of FIG. 8A, in which the parameters were adjusted so that the mass-resolving power would improve in the high-mass range, the mass-resolving power deteriorated (i.e. the peaks were broader) in the middle-mass range (from m/z652 to m/z1225). In the example of FIG. 8B, in which the parameters were adjusted so that the mass-resolving power would improve in the middle-mass range, the mass-resolving power deteriorated in the high-mass range. Furthermore, although the mass-resolving power was high in the middle-mass range, the ion sensitivity in this range was considerably deteriorated. In the example of FIG. 8C, a diode capable of operating with high linearity was used in the wave-detection circuit, and the parameters were adjusted so that the mass-resolving power would be high over the entire mass range. This situation can be regarded as almost ideal. However, a diode with which this situation can be realized is difficult to procure and extremely expensive as compared to the normal type of diodes.